A new type of electrochromic hydride material has
interesting and unusual properties. Thin Ni-Mg films, for example, are
mirror-like in appearance and have very low visible transmittance. On
exposure to hydrogen gas or on reduction in alkaline electrolyte, the
films become transparent. The transition is believed to result from
formation of nickel magnesium hydride, Mg2NiH4.
Switchable mirrors based on rare earth hydrides were discovered in
1996 at Vrije University in the Netherlands, Rare earth-magnesium
alloy films were subsequently found to be superior to the pure lanthanides
in maximum transparency and mirror-state reflectivity by Philips
Laboratories. The newer transition-metal types which use less expensive
and less reactive materials were discovered at LBNL. This has now become a
very active area of study with a network
of researchers.
Unlike
conventional oxide electrochromics, hydrides become reflective rather than
absorbing. The dynamic range is typically 10 times greater than for
absorbing materials. This has potential advantages in terms of energy
performance, temperature stability, glare control and privacy. The
films were deposited using cosputtering from separate targets of Ni and Mg
to facilitate variations in composition. Single-target d.c. magnetron
sputtering could be used eventually which would be relatively simple
compared to deposition of electrochromic oxides .
